It is interesting that a critique against proponents of non-violence is that they're supposedly "selfish".

It takes selfishness to consider that killing others is okay, including to protect one's own self. From a buddhist perspective, self-defence is off. Attachment to the self leads to fear of loss, which easily leads to violence. No difficulty there.
But the defence of others, putting oneself in harm's way for the benefit of others, might be ethically valid. Putting oneself in harm's way for the benefit of others might be a manifestation of selflessness ('might': it depends very much on whether one expects some reward —earthly or in the after life— for the good deed, or simply responds appropriately to what the situation requires). www.koan.mu/interpreting_the_teachings.htm

So, is it selfish not to help allies?

Helping allies is what's 'selfish'. It is simply another form of 'self' ('my' alliance vs. others). To even have 'allies' requires to project qualities on a person (or a group of persons) rather than discern the qualities from their acts. It is a sign of mis-attribution: what might be wholesome is how one functions, what one does (or refrains from doing…), not who one 'is'.

Alliances are very much what led to the latest, ultra-misguided Irak war, based on self-serving lies, with Americans calling on their allies so their aggression would have some legitimacy by virtue of "an international coalition."
The countries who responded to such a call projected qualities on the US (as an entity) rather than on their acts (as a process).
This is the very basis of 'ignorance', of unhelpful discernment. This confused the wholesomeness of some American acts (e.g. during WWII) with a wholesomeness of the US in and of themselves. One should certainly back up the US when they act wholesomely (with wise discernment of cause and effect, with intelligence, with wisdom, for the benefit of all, not just themselves), but not when they don't.

Alliances should be like 'vows', they should be alliances with virtues, not groups of people. Then, when one group behaves wisely, it benefits from our assistance regardless of who they are and of the past; and vice versa.
This is the very basis of resistance to barbary too: resistance in Germany or in collaborationist France (Vichy's government) was based on rejecting the artificial 'safety' of the group (turning a blind eye on the barbary committed by said group, for the sake of belonging, of staying low, of self-preservation), staying true to higher values instead.
Contrarily to the common views based on retaliation and punishment, it makes particular sense to support groups with a bad past when they tentatively act ethically… because it helps them transition to a more ethical way of living, which will benefit all (us included)! To call upon mistrust (based on the past) to withhold assistance is to push them back towards their old habits. We support the transition, by setting positive incentives for wholesome speech, wholesome act, wholesome livelihood! And carrot always works better than stick.
On the other hand, we don't actually 'support' allies when we turn a blind eye on their current ignorance, some weak-at-the-knee 'forgiveness' based on past wholesomeness: if anything, we need to call those who had wholesome moments in the past to continue cultivating wholesomeness, and to refrain from the unwholesome. Good deeds are not brownie points you save so you can act unethically later and get away with it!
"Right effort" is described in fourfold approach: give rise to the wholesome, support the wholesome, weaken the unwholesome, cease the unwholesome.
Germany was asked to pay huge reparations after WWI, and this indirectly led to WWII. (West) Germany wasn't asked to pay huge reparation after WWII, and Europe has benefitted from one of the longest period of peace in History since. (East) Germany was asked to pay reparations into the USSR and not only didn't strive but was where the Iron Wall first fell! Retaliation doesn't work.

To present non-violence as selfish could make sense if politics were aligned with virtues and behaviours, rather than with self-interests and entities: processes over entities.
In the world as it is now, to present non-violence as selfish is just a manipulative attempt to generate a "guilt trip" and thus get people (who care not to hurt others based on selfish interests and delusions) to take sides a priori, i.e. to strengthen prejudices, to build an artificial self, an arbitrary separation between one group and the other (not based on facts but on assumptions about the future).

If we accept that Buddhism 'might', under conditions, be okay with defending others, even though Buddhism does not support self-defence, then we have to keep in mind that a wholesome defence of others would only arise based on the context at hand: it calls neither for premeditation, nor prejudices, nor preconceptions on who is 'friendly' and who's not.
Just like engaging with "dharma friends" does equate cultivating virtues together, not clinging to specific people, titles, etc.
The appropriate defence of others would be an embodiment of supporting the wholesome and weakening the unwholesome in the moment, based on processes unfolding in the moment, neither on the past nor on 'permanent' entities.